I have a Hyundai Elantra , I want to add a AUX in for the car CD headunit. The modle of the CD headunit is H260 with a 13-pin interface(1 CDC R(+)\2 CDC R(-)\3 CDC L(-)\4 CDC L(+)\5 CDC TX(DO)\6 CLK\7 CDC RX(DI)\8 D-GND\9 BACK UP(+)\10 A/GND\11 P-GND\12 N.C\13 ACC B(+)). It seems use SPI to connect headunit and CD changer.
I have a problem of the voltage, the voltage between ACC and D-GND is 7V, but if connect ACC and D-GND with a 7805, the voltage will change to 3V.The voltage between ACC and P-GND is 12V, the voltage between ACC and CLK is 12V, but the voltage between CLK and D-GND is -3V.